Cathode connection for a reduction cell



Feb. 20; 1968 J. J; VADLA CATHODE CONNECTION FOR A REDUCTION CELL Filed Oct. 25, 1963 A KL D WA A T TORNE V United States Patent 3,369,986 CATHODE CONNECTION FOR A REDUCTION CELL Jostein J. Vadla, Grand Island, N.Y., assignor to Union Carbide Corporation, a corporation of New York Filed Oct. 23, 1963, Ser. No. 318,321 6 Claims. (Cl. 204-243) ABSTRACT OF THE DISCLOSURE A connection for a reduction cell of the type used in the electrolytic production of aluminum characterized by the use of a collector bar having a greater width than height in cross section. The use of such a specifically shaped collector bar causes a reduction in the electrical resistance for the overall connection. A plurality of collector bars spaced apart at the central portion of the reduction cell provide additional efficient operation.

This invention relates to an improved cathode connection for a reduction cell used in molten salt electrolysis such as an aluminum pot or cell.

In general, aluminum reduction cells comprise an anode suspended into a pot having a carbonaceous bottom, side and end walls contained and insulated in a steel shell. Current enters the reduction cell through the anode and then is made to pass through a layer of molten salt bath into a molten aluminum metal cathode. From the aluminum cathode the current enters the carbonaceous bottom of the pot and is carried to an external bus bar system by means of a plurality of steel collector bars embedded in the carbonaceous bottom.

The present invention is particularly directed to the carbonaceous bottom lining and the collector bars. It is conventional in the aluminum industry to provide a cathode connection wherein a substantially square cross-sectional steel collector bar is disposed in a double-dovetail slot provided in the carbonaceous bottom block in such a manner that a clearance is provided on all sides thereof. This clearance is filled with a molten cast iron or in some cases with a conductive carbonaceous ramming paste. A typical aluminum pot bottom lining contains a plurality of carbonaceous bottom blocks, each one of which extends the full width of the pot bottom; each block normally contains one collector bar which extends beyond both sides of the pot and is suitably connected to the external bus bar system.

One of the disadvantages of this system is the necessity of having to have the cast-in connection made at a foundry or else having to maintain foundry facilities at the aluminum plant, either of which is expensive. Although a substantial portion of this expense can be obviated by using a rammed conductive carbonaceous paste instead of the cast iron, this construction is associated with substantially greater power losses due to the higher resistivity of the ramming paste. Another disadvantage of the conventional aluminum pot will be seen by a consideration of the resistivity of the various elements of the pot as will be fully discussed hereinafter.

It is the principal object of the invention to provide a novel and improved cathode connection which overcomes the disadvantages of the prior art connections.

Another object of the invention is to minimize the power loss through the collector bar-bottom portion of the reduction cell.

Yet another object is to provide a cathode connection which is economical and which is relatively simple in construction.

These and other objects and advantages of the invention are accomplished by a cathode connection for a reduction cell comprising a carbonaceous bottom block having at least two rather wide and relatively shallow slots which extend from opposite edges in an in line fashion toward each other but which do not enter the center portion of the block. The slots contain at least two collector bars.

-In the drawings:

FIGURE 1 is an elevational view of a typical aluminum reduction cell embodying the invention; and

FIGURE 2 is a sectional view along the lines 22 of FIGURE 1 showing the current carrying portion of the bottom block-collector bar assembly.

As shown in FIGURE 1, the cell 10 comprises a steel shell 12, insulation 14, carbonaceous side walls 16 and 18 and carbonaceous bottom blocks 20. One or more anodes 22 are suspended above the bottom blocks 20. The upper end 244 of the anode 22 is suitably connected to an external bus bar system (not shown) and the lower end 26 is spaced apart a predetermined distance from a molten aluminum cathode 28. The space between the anode 22 and the aluminum cathode 28 is filled with a molten electrolyte 30 into which is dissolved aluminum oxide. The electrolyte 30 forms a frozen crust 32 along the side walls 16 and end walls (not shown) of the cell 10.

As best shown in FIGURE 2, a rather wide and thin collector bar 34 is placed within a correspondingly shallow slot 36 provided in the bottom block 20. The collector bar 34 and slot 36 are preferably dove-tail in shape and a small clearance 38 is provided for along the longitudinal edges 40 and 42 of the collector bar 34. This clearance 38 compensates for the differential in CTE (coefficient of thermal expansion) between the collector bar 34 and the bottom block 20 and it may be suitably filled up with a conductive carbonaceous paste 35, preferably a thermosetting one. As shown in FIGURE 1, the center portion 37 of the slot 36 does not contain the collector bar 34. This portion 37 of the slot 36 is preferably filled with ramming paste 35 or an insulating material although it may remain as a void if desired. Of course, it is apparent that the slot 36 need not extend completely across the bottom block 20. Two slots may be machined from the edges 44 and 46 of the block 20 toward each other such that they are in line but are spaced apart from each other by an unmachined center portion of the bottom block 20.

In order to more fully appreciate the invention and to recognize the disadvantages of the prior art, the current path through the current carrying elements of the pot imposed by their typical resistivities are illustrated by the dotted path lines of FIGURE 1 and also typical resistivities of the various elements of a pot are tabulated below in Table I.

Table I Element: Resistivity (ohm-cm.) Anode 5 10- Bath 0.35 Molten aluminum 3 X 10- Carbon 4X10 Graphite 1X 10 Steel 1 10 Because of bi-lateral symmetry of the reduction cell, only one side of the drawing shows the current paths. Current from the anode entering the high resistance bath is normalized through the bath to a high degree of uniformity. Upon entering the low resistance molten aluminum layer, the elements of current will tend to initially follow lines parallel to the interface between bath and molten metal such that the current is forced to travel through the molten metal to a point some distance away from the center line of the cell before entering the carbonaceous bottom lining. The current path will again be substantially normalized through the carbonaceous bottom and exits straight through the steel collector bar. It will be noticed, as best shown by the path lines of FIGURE 1 that the portion of the steel bar substantially below the center line of the anode carries no current and hence serves no useful purpose.

Since power loss is also a function of current density, it should be readily apparent from a study of FIGURE 2 that an essentially square cross-sectional collector bar would distort the current distribution within the block and gives a high current density near the contact area (essentially the upper surface of the bar). The power loss of the conventional constructions will therefore be higher than necessitated by the cross-section of the steel collector and the carbonaceous block.

In order to show that the cathode connection of the invention is superior to the existing conventional connections, resistance measurements were made at operating temperatures (9001000 C.) for both the connection of the invention and for connections heretofore used. Since the term carbonaceous includes both carbon and/ or graphite measurements were made employing carbon bottom blocks as well as graphite bottom blocks.

Three diiferent types of reduction cell bottoms were tested. One was a conventional carbon bottom block embodying a substantially square cross-sectional collector bar. Another test was conducted using a carbon bottom block and the collector bars of the invention and the last test employed a graphite bottom block and the collector bars of the invention. The following data set forth is typical of the resistances across the collector bar to the top surface of the carbon or graphite bottom block. The resistance of the path through the carbon or graphite The resistance values measured on a conventional carbon bottom block and cast in collector bars agree well with the measurements made on starting reduction cell as is well known in the art or as reported by literature.

It can be seen that the total resistance of the cathode connection of the invention whether embodying a carbon or graphite bottom plate is substantially lower than the total resistance exhibited by a conventional bottom platecollector bar construction. It is further seen that this reduction in resistance is achieved partially by an improvement of the contact per se and partially by the resistance of the path through the bottom block. The reduction of the resistance of the path through the bottom block is due entirely to the increase in the average cross-sectional area of the current path as imposed by the wider current collection bar of the invention. In the case of carbon, it is noted that the specific resistance of the bottom block itself remain unchanged.

'In conclusion, it is believed clear that this type of cathode connection is relatively simple and economical to manufacture and it exhibits improved characteristics over prior art cathode connection.

I claim:

1. A connection for a reduction cell of the type used in the electrolytic production of aluminum, comprising, an anode suspended within a pot having side and end walls and a carbonaceous bottom block, adjacent thereto, said carbonaceous bottom block being provided with a slotted portion, and at least one collector bar positioned in the slotted portion of said carbonaceous block, said collector bar having a cross sectional area which is characterized by having a width dimension greater than the height dimension and having a dovetail shape, the wider surface of said dovetail shaped collector bar being positioned nearest said anode whereby a reduction in electrical resistance through said connection is achieved.

2. A connection of claim 1 wherein a pair of collector bars are positioned in said slotted portion and are spaced apart from each other directly below said anode.

'3. The connection of claim 2 wherein the segment of the slotted portion in said carbonaceous bottom block between said collector bars is filled with a conductive carbonaceous ramming paste.

4. The connection of claim 1 wherein said slotted portion and said collector bar are interlockingly secured in a dove-tail connection.

5. The connection of claim 1 wherein said bottom is composed of graphite.

6. The connection of claim 1 wherein said bottom is composed of carbon.

References Cited UNITED STATES PATENTS 2,593,751 4/ 1952 Grolee 204243 2,874,110 2/ 1959 Thayer 204243 3,179,736 4/1965 Ramsey 204243 HOWARD S. W IL'LIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

E. ZAGARELLA, Assistant Examiner. 

